The generation of hydrogen sulfide (H2S) results in a variety of corrosion problems. For example, sulfidogenesis results in a variety of oil recovery problems, including oil reservoir souring, contamination of crude oil, metal corrosion, and the precipitation of metal sulfides which can subsequently plug pumping wells.
An important aspect of microbial enhanced-hydrocarbon recovery (MEHR) is control of reservoir souring. Reservoir souring is characterized by significant increases in H2S in production gas and soluble HS− in production fluids, typically after initiation of secondary recovery processes involving water injection. Although several abiotic mechanisms have been proposed as the cause of reservoir souring including thermochemical sulfate (SO42−) reduction and pyrite (FeS2) dissolution, it is now widely accepted that sulfate-reduction by dissimilatory sulfate-reducing bacteria (SRB) is primarily responsible for sulfide production in reservoir souring as a result of water flooding (Vance and Thrasher, Petroleum Microbiology, eds B. Ollivier & M. Magot, ASM Press, 2005).
Sour service metallurgy for wells, pipelines, and pump systems carry an estimated cost premium of 2% of total project costs at project initiation but may be an order of magnitude higher if retrofitting is required (Al-Rasheedi et al., SPE Middle East Oil Show, Society of Petroleum Engineers). Sour production facilities also entail additional costs associated with prevention of operator exposure to toxic H2S; control of oil-wet iron sulfide pads that reduce oil-water separator performance, management of iron sulfide solids that interfere with produced water cleanup and recycle, and accumulation of iron-sulfide deposits that may foul equipment and enhance equipment corrosion. In addition, revenue loss may result from limitations imposed on pumping high volumes of oil and gas with excessive H2S concentrations through export lines to ensure system integrity (Vance and Thrasher, Petroleum Microbiology, eds B. Ollivier & M. Magot, ASM Press, 2005).
Effort has focused on mechanisms by which H2S generation from dissimilatory sulfate-reducing metabolism can be inhibited. Significant research has focused on thermodynamic inhibition of SRB activity by the addition of nitrate to the injection waters. Thermodynamic considerations indicate that microbial nitrate reduction is energetically more favorable than Fe(III)-reduction, sulfate-reduction, or methanogenesis and should therefore occur first (Coates and Achenbach, Manual of Environmental Microbiology, eds C. J. Hurst et al., 719-727, ASM Press, 2001; and Lovely and Chapelle, Reviews of Geophysics 33, 365-381, 1995). For example the Gibbs free energy for the anaerobic degradation of toluene coupled to nitrate-reduction (ΔGo′=−3,554 kJmol−1 toluene) is significantly higher than that coupled to sulfate-reduction (ΔGo′=−205 kJmol−1 toluene). Thus, the addition of excess amounts of nitrate should result in the preferential utilization of this electron acceptor and the selective inhibition of sulfate-reduction.
However, thermodynamic preferential use of nitrate over sulfate is not mutually exclusive in a system unlimited for electron donors, such as in an oil reservoir where hydrocarbon reserves represent an inexhaustible supply of biodegradable carbon to active microbial communities (Coates and Achenbach, Manual of Environmental Microbiology, eds C. J. Hurst et al., 719-727, ASM Press, 2001; and Van Trump and Coates, Isme J 3, 466-476, 2009). As such, while the presence of nitrate will slow down sulfate-reduction, it will not completely inhibit sulfate metabolism. Furthermore, the results of previous studies suggest that addition of a thermodynamically more favorable electron acceptor, such as Fe(III), may not be enough to completely inhibit sulfate-reduction once an active SRB community is established (Coates et al., Environmental Science and Technology 30, 2784-2789, 1996).
Thus, there exists a need to develop an economic and efficient method of regulating the amount of S2− produced by microbial sulfate-reduction in systems such as during oil recovery.